Physical one-way functions have already been proposed as a basis for cryptography, copy protection or identification cards. According to the known prior art, a diffractive structure is illuminated with an arbitrary wave front, and the resulting speckle pattern is detected by a detector, for instance a CCD camera. From the speckle pattern a cryptographic key is created, which uniquely identifies the diffractive structure. The robustness of such a system depends strongly on the alignment between the diffractive structure and the detector.
The paper entitled “Physical one-way functions”, Science, 20 Sep. 2002, p. 297 shows the use of diffractive structures for realizing one-way functions for cryptography purpose. Said paper is based on the detection of the speckle pattern by a high resolution CCD detector, which is rather expensive. And some problems can be expected from alignment between the light source, the diffractive structure and the detector.